10 Recommender Systems Interview Questions and Answers for ML Engineers

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1. What is Collaborative Filtering?

Collaborative filtering is a type of recommender system that predicts what items a user might like based on the preferences of similar users. It works by analyzing user behavior and finding patterns that can be used to make recommendations.

One common approach to collaborative filtering is user-based filtering, where the system identifies users with similar preferences and recommends items that those users have liked in the past. Another approach is item-based filtering, where the system recommends items that are similar to those that the user has already liked.

For example, let's say we have a dataset of movie ratings from different users. Using collaborative filtering, we can recommend movies to a user based on the ratings of similar users. If User X has given high ratings to action movies and low ratings to romantic comedies, the system will recommend action movies to User X and avoid recommending romantic comedies.

Collaborative filtering has been used successfully in many real-world applications, such as movie recommendations on Netflix and product recommendations on Amazon. In fact, a study by The Royal Society found that collaborative filtering improved the accuracy of recommendations by an average of 35% compared to traditional approaches.

2. What is Content-Based Filtering?

Content-Based Filtering is a type of recommender system in which the recommendations are based on the similarity between the content of the items being recommended and the content of items the user has liked or consumed in the past. This approach builds a model that represents the user’s preferences based on item features.

A classic example of content-based filtering is the “related items” feature in online marketplaces. For example, if a user liked a smartphone, a content-based recommender system would recommend other smartphones with similar features, such as a large screen size, high resolution, and fast processor.

One of the advantages of content-based filtering is that it does not require user information or preferences. This makes it particularly useful for cold-start problems: recommending items when there is no information available about the user.

However, one limitation of content-based filtering is the difficulty of representing each item accurately. For example, if the content of an item is described with text, the model might struggle to capture the meaning of the text in a meaningful way. Another drawback is that content-based filtering tends to recommend items that are similar to those a user has already consumed, which can limit the diversity of recommendations.

One way to mitigate this problem is to incorporate a hybrid approach that combines different types of recommender systems. For instance, using both content-based and collaborative filtering increases the accuracy and diversity of recommendations.

3. What is hybrid Recommender Systems?

A Hybrid Recommender System combines two or more recommendation techniques in order to achieve better accuracy and coverage in the recommendations. The two main types of systems used in hybrid models are Collaborative Filtering and Content-Based Filtering.

Collaborative Filtering uses data on user behavior, such as ratings or clicks, to recommend items based on the preferences of similar users. Content-Based Filtering uses data on the features of the items, such as genre or topic, to recommend items based on the interests of the user.

One example of a hybrid recommender system is the Netflix recommendation system. Netflix uses collaborative filtering to suggest movies based on similar users' preferences, but also incorporates content-based filtering by suggesting titles based on the genre, actor, or director that the user has previously viewed.

The benefit of using a hybrid approach is that it can overcome the limitations of individual techniques by combining their strengths. For example, content-based systems may struggle to recommend new and unique items, whereas collaborative filtering can solve this problem by leveraging the behavior of similar users.

  1. Hybrid Recommender Systems offer better recommendations with higher accuracy and coverage.
  2. They combine the strengths of multiple recommendation techniques.
  3. The Netflix recommendation system is an example of a hybrid model.
  4. Collaborative Filtering and Content-Based Filtering are the two main types of systems used in hybrid models.

4. What are the different evaluation metrics for Recommender Systems?

Evaluation Metrics for Recommender Systems:

Recommender System is a type of machine learning system that predicts and recommends the most relevant items to the users based on their preferences, browsing history, and other data. Evaluating recommender systems is an essential part of building them, as it helps us understand how well they are performing. Below are some of the commonly used evaluation metrics:

  1. Accuracy: Accuracy is a common metric used to evaluate the performance of recommender systems. It measures the proportion of correct predictions against total predictions. For example, if a recommender system predicts 8 out of 10 relevant items, then its accuracy is 80%.
  2. Precision and Recall: Precision measures the proportion of relevant items recommended by the system that are actually relevant to the user. Recall measures the proportion of relevant items that are recommended by the system. Both Precision and Recall are important as they give us a better understanding of the performance of the system.
    • Precision: If the recommender system recommends 10 relevant items to a user, and 8 of them are indeed relevant, then the precision is 80%.
    • Recall: If there are 20 relevant items for a user, and the recommender system recommends 10 of them, then the recall is 50%.
  3. F1 Score: F1 Score is the harmonic mean of Precision and Recall, and it is a useful metric for evaluating the overall performance of the recommender system. It ranges between 0 and 1, where 1 is the best score.
  4. Mean Absolute Error (MAE): MAE measures the average difference between the predicted and actual ratings. It is also known as the L1 norm, and the lower the MAE, the better the performance of the system.
  5. Root Mean Squared Error (RMSE): RMSE is another commonly used metric for evaluating recommender systems. It measures the square root of the average squared difference between the predicted and actual ratings. The lower the RMSE, the better the performance of the system.

Overall, the selection of an evaluation metric will depend on the type of recommender system being built and the specific requirements of the project. It is important to choose the most appropriate metric based on these factors to achieve the desired performance.

5. What is Matrix Factorization?

What is Matrix Factorization?

Matrix Factorization is a technique used to predict user preferences or item ratings in recommender systems. It involves breaking down a large matrix of user items into smaller matrices, representing latent factors that underlie the interactions between users and items. These latent factors could be anything, such as genre or director, in the case of movie ratings, or brand or category in e-commerce sites.

Matrix Factorization produces a low-dimensional representation of users and items that allows for better predictions of unknown entries in the matrix. By doing so, it helps to overcome the sparsity problem that is common in recommender systems where users only rate a few items.

To illustrate this technique, let's consider a simple example of a movie rating matrix. Suppose we have five users who have rated four different movies. The matrix would look something like this:

  • User/Movie Movie A Movie B Movie C Movie D
    User 1 5 3 2 0
    User 2 0 1 0 4
    User 3 4 0 5 0
    User 4 2 0 3 2
    User 5 0 0 1 3

Each cell in the matrix represents a rating given by a user to a movie, with zero indicating no rating. To factorize this matrix, we would decompose it into two matrices, one representing users and the other representing movies. We then multiply these matrices to obtain a low-dimensional representation of the original matrix. By doing so, we can fill in missing values in the matrix with predicted ratings.

For example, suppose we decompose the matrix into two latent matrices, one representing users and the other representing movies, with four latent factors. We can then get a predicted rating for user 1 on movie D by multiplying the user-factor vector for user 1 (5, 2, 4, 3) with the movie-factor vector for movie D (0.5, -0.1, 1.2, 0.6), taking the sum and adding a bias term. The resulting predicted rating is 1.5.

Matrix Factorization is a powerful technique for improving the accuracy of recommender systems, and it has been used in many real-world applications, such as Netflix movie recommendations and Amazon product recommendations.

6. What is Singular Value Decomposition?

Singular Value Decomposition or SVD is a matrix factorization method used in recommendation systems to discover underlying patterns between users and items. It works by decomposing a large matrix into smaller matrices to simplify computation and improve prediction accuracy.

Consider a ratings matrix where rows represent users, columns represent items and the values represent the rating of a user for an item. Let's assume a ratings matrix of size (10000, 5000) with 10 million ratings. Instead of using this large matrix, we can use SVD to break it down into three smaller matrices U, Σ and V:

  1. The U matrix represents how users are related to each other based on their ratings.
  2. The Σ matrix is a diagonal matrix that represents how much each feature matters.
  3. The V matrix represents how items are related to each other based on their ratings.

The SVD algorithm results in the decomposition of the ratings matrix into the product of these three matrices:

R = U x Σ x V^T

Once we have these smaller matrices, we can use them to make predictions. By taking the dot product of the U, Σ and V matrices, we can approximate the rating that a user might give to an item that they have not rated before. For example, if user 1234 has not rated item 5678, we can predict a rating of 4.5 based on the patterns found in the U, Σ and V matrices.

Through SVD, we have turned a large, complex matrix into smaller, simplified matrices that we can use to make predictions with better accuracy, which is a key to success of recommendation systems.

7. What is Alternating Least Squares?

Alternating Least Squares (ALS) is a popular algorithm used in collaborative filtering. It is designed to factorize the user-item interaction matrix, decomposing it into two low-rank matrices: a user matrix and an item matrix.

The factorization performs matrix completion, which helps to recommend items to users based on their past interactions. The ALS algorithm alternates between fixing one of the matrices and optimizing the other matrix to minimize the squared error loss function.

ALS has many applications, including in the movie recommendation system. For instance, suppose we have a dataset of movies and users who have rated them on a scale of 1-5. Using ALS, we can recommend movies to users based on their preferences.

Here is an example:

  1. Suppose the user matrix is of dimension (1000 x 10) and the item matrix is of dimension (10 x 500).
  2. We can multiply the user matrix and the item matrix to get the predicted rating matrix of dimension (1000 x 500).
  3. We then compare the predicted matrix with the actual ratings matrix, which is of the same dimension.
  4. Using the squared error loss function, we minimize the difference between the predicted and actual ratings.
  5. The minimized matrix gives us the recommended movies for users.

Overall, the ALS algorithm is an effective means of building recommender systems that require matrix factorization.

8. What is Stochastic Gradient Descent?

Stochastic Gradient Descent (SGD) is an optimization algorithm that is commonly used in machine learning for training artificial neural networks. It is a variant of regular gradient descent that is often used when dealing with large datasets.

Instead of computing the gradient of the cost function over the entire training set, SGD randomly selects a small batch of training samples and calculates the gradient of the cost function with respect to those samples. This batch is then used to update the parameters of the model.

Because SGD only considers a small subset of the training data at each iteration, it converges faster than regular gradient descent. However, the convergence is more noisy and may require more iterations to reach a minimum.

Here is an example of how SGD can be used to train a logistic regression model:

  1. Initialize the weights of the logistic regression model to random values.
  2. Select a small batch of training samples.
  3. Calculate the gradient of the cost function for the selected batch.
  4. Update the weights of the model using the gradient and a learning rate.
  5. Repeat steps 2-4 until convergence or a maximum number of iterations is reached.

SGD has been shown to be very effective in training deep neural networks, such as convolutional neural networks, for image recognition tasks. One example is the ImageNet Large Scale Visual Recognition Challenge, where the winning team used a deep convolutional neural network trained with SGD to achieve state-of-the-art results on a large-scale image classification task.

9. Can you explain the difference between implicit and explicit feedback?

Implicit feedback is feedback that is not given directly by the user, but rather is inferred based on the user's behavior. For instance, if a user frequently listens to a particular artist on a music streaming platform, that can be considered as implicit feedback as it indicates that the user likes that artist.

Explicit feedback is feedback that is directly given by the user. For instance, a user rating a product on e-commerce platform is considered as explicit feedback as it directly states the user's opinion about the product.

The main difference between the two is the level of user engagement and the amount of information available. Implicit feedback is generally passive and does not require any active input from the user. It is also often noisy and ambiguous, making it more difficult to interpret. Explicit feedback on the other hand, is more direct and explicit, making it easier to interpret and analyze.

In a recommendation system, both kinds of feedback can be used to make recommendations. Explicit feedback can be used to directly infer user preferences and to train a model to make better recommendations. On the other hand, implicit feedback can be used to infer user preferences indirectly, and to provide additional information to the recommendation algorithm.

For example, in a movie recommendation system, explicit feedback might be ratings that users give to movies, whereas implicit feedback might be the frequency at which they watch certain genres of movies. By combining both kinds of feedback, the recommendation algorithm can provide more accurate and personalized recommendations.

10. Can you outline the steps to build a Recommender System?

  1. Define the problem: Identify the type of Recommender System that best suits the problem. For example, if the need is to recommend a set of products to a user, then a Collaborative Filtering-based Recommender System is the ideal choice.
  2. Gather and preprocess the Data: Collect sufficient and adequate data on Users and Items, with relevant metadata. Carry out Data cleaning, Preprocessing and Feature Engineering (if necessary).
  3. Split the Data: Divide the Preprocessed data into Training, Validation and Test sets. The sizes can vary depending on the size of the dataset but typically 70-20-10% is good.
  4. Select appropriate Metrics: Decide on the relevant Evaluation Metrics to measure the performance of the Recommender System.
  5. Develop the Model: Develop and fine-tune a suitable Modelling approach based on the Split data, the Type of Recommender System, and the Evaluation Metrics. For example, a Matrix Factorization approach using Gradient Descent could be used for Collaborative Filtering-based Recommender Systems.
  6. Train the Model: Train the Model on the Training data and Validate the Model on the Validation data, adjusting the hyperparameters if necessary.
  7. Assess Model Performance: Test the performance of the Model on the Test data using the Evaluation metrics previously defined. Output the final results, such as Precision or Recall, to determine which Model performs the best in production.
  8. Deploy the Model: After choosing the best Model, deploy it for use in Production.
  9. Maintain the Model: Maintain the Model by periodically retraining, as necessary, based on newly connected data.
  10. Iterate: Iterate on the entire process to improve the Model's accuracy and efficiency continually.

For Example, Yelp's Recommender System, which makes personalized restaurant reviews for individual users by forming top-N recommendations via matrix factorization, used the above steps to train its Model. It used hundreds of thousands of reviews to recommend the best restaurants based on user preferences.


Recommender systems are an essential part of many tech companies today, and ML engineers play a critical role in creating and maintaining them. If you're preparing for an interview as an ML engineer, these ten questions and answers should help you feel more confident and prepared.

However, the job search process doesn't end with the interview. It's essential to write a great cover letter to showcase your skills and experience to potential employers. Here is a guide to help you write a compelling cover letter.

You should also prepare an impressive ML engineering CV to showcase your professional experience and accomplishments. Here is a guide that can help you create a standout CV.

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